As a successor of a WCDMA (Wideband Code Division Multiplexing Access) system, an HSDPA (High-Speed Downlink Packet Access) system, and an HSUPA (High-Speed Uplink Packet Access) system, an LTE (Long Term Evolution) system has been considered and standardized by 3GPP (The 3rd Generation Partnership Project), which is a standardization organization of WCDMA.
Furthermore, as a successor of the LTE system, an LTE-advanced system is under consideration by 3GPP. The requirements for the LTE-advanced system are summarized in TS 36.913 (V8.0.1).
As one of the requirements in the LTE-advanced system, an agreement is reached that carrier aggregation is applied. When carrier aggregation is applied, a mobile station UE can receive downlink signals simultaneously using plural carriers or transmit uplink signals simultaneously using plural carriers. Each carrier used in carrier aggregation is referred to as a “component carrier”.
The plural component carriers are categorized into a primary component carrier as a main carrier and one or more secondary component carriers other than the primary component carrier.
When a mobile station UE performs communications always using the primary component carrier and the secondary component carriers, a problem arises that power consumption becomes higher in proportion to the number of component carriers. As used herein, communicating using the primary component carrier and the secondary component carriers includes usual data transmission and reception, cell search or measurement on the respective carriers, and radio link monitoring.
For example, the cell search includes establishing synchronization in downlink using downlink synchronization signals in a serving cell and an adjacent cell. Since cell search is the processing for detecting a destination cell (target cell) while a mobile station UE is moving, the mobile station UE periodically needs to perform cell search. For example, the measurement includes measuring received power (more specifically, RSRP (Reference Signal Received Power) or the like) of reference signals in a serving cell and an adjacent cell. It should be noted that the combined processing of cell search and measurement may be referred to as “measurement”. The radio link monitoring includes measuring radio quality (more specifically, SIR (Signal-to-Interference Ratio)) of reference signals in a serving cell, determining whether the SIR is above a predetermined threshold, and determining that the serving cell is in out-of-synchronization when the SIR is below the predetermined threshold. The processing associated with cell search, measurement, and radio link monitoring and their performance definitions are described in 3GPP TS 36.213 V8.8.0 (2009-09) and 3GPP TS 36.133 V8.7.0 (2009-09), for example.
In order to address the problem of power consumption, it is considered that control of activation/de-activation is applied in a secondary component carrier, for example. For example, on a secondary component carrier in a de-activation state, a mobile station UE does not perform usual data transmission and reception and reduces the frequencies of cell search, measurement, and radio link monitoring, thereby saving the battery. The processing of de-activation on a secondary component carrier is performed when the amount of data to be communicated is small, for example.
In the LTE system, discontinuous reception (DRX) control is applied in order to save the battery in a mobile station UE (see 3GPP TS 36.321 V8.7.0 (2009-09)). Discontinuous reception control in the LTE system is applied when a radio base station eNB and a mobile station UE are in a connected state and there are no data to be transmitted. A mobile station UE in a discontinuous reception state is configured to periodically, that is, intermittently, receive downlink control signals transmitted via a PDCCH (Physical Downlink Control Channel). The duration in which the downlink control signals are received via the PDCCH is referred to as an “on-duration” (ON interval or reception interval). Typically, cell search, measurement, and radio link monitoring are performed in the on-duration. Since a mobile station UE need only receive downlink signals transmitted via the PDCCH intermittently, rather than at all timings, and thus needs only to intermittently perform cell search, measurement, and radio link monitoring, power consumption of the battery can be reduced.
More specifically, as shown in FIG. 1, a mobile station UE is configured to perform reception of the PDCCH, cell search, measurement, radio link monitoring, and so on only during a reception interval (5 ms in the example of FIG. 1) provided for each DRX period (1280 ms in the example of FIG. 1) and turn off the transceiver during the other interval. As a result, the mobile station UE can reduce power consumption of the battery.
In addition, in the LTE system, a measurement gap is defined in order to perform measurement on a carrier with a different frequency or a carrier for a different radio communication system (see 3GPP TS 36.331 V8.8.0 (2009-12)). The length of the measurement gap is defined as 6 ms and its periodicity is defined as 40 ms or 80 ms, for example. During the measurement gap, a mobile station UE suspends communications in a serving cell and performs measurement on a carrier with a different frequency or a carrier for a different radio communication system.